Test driving a trio of Kenworths

Getting stuck is frustrating and embarrassing. Here I was, at Eaton Corp.’s proving grounds near Marshall, Mich., test-driving a trio of Kenworths with the latest Paccar-branded automated transmissions.

Things were going well until I drove a T880 vocational tractor with its heavy-laden flatbed trailer into a sandpit. A little while earlier, I had taken a T480 truck through it just fine, so my host, Kenworth’s Alex Reid, had suggested steering this 82,000-pound semi into another section of the pit. We got maybe 50 feet and there we bogged down, axle deep. Trying to rock out only dug the wheels in deeper.

I barked something like “Shucks!” while Reid got on his hand-held radio and called for help. He is Kenworth’s vocational product marketing manager and knows quite a lot about what these trucks with their Eaton-sourced self-shifting gearboxes can do. A mechanical engineer by trade, he was involved in their development with Eaton counterparts. So he was surprised that we were stymied but surmised that the trailer’s unpowered wheels had grabbed at the sand and fatally impeded our progress. Also, this part of the pit was deeper and wetter than it looked.

Answering Reid’s radioed SOS, a crew led by Alison Riechow, the track supervisor, appeared in a Ford pickup carrying a strong tow strap and a pair of bulky hook-up ropes. Reid helped rig them to the trailer’s rear axle, and Riechow’s assistant attached the other end to the pintle of a Cat front-end loader that also had trundled onto the scene. In minutes, the Cat’s operator eased the rig backward out of the sand, and it was ready for more adventures.

Paccar labels these transmissions “TX” and mates them with its own MX diesels, and since 2018 has offered them in 12- and 18-speed versions in Kenworths and Peterbilts.

These go in on-highway vehicles. More recently came Pro models for vocational use with wide-spread ratios, extreme-duty clutches, PTO mounts and special calibrations in their electronic controls. With decent traction, the TX-18 Pro in the T880 tractor performed well, and a TX-12 Pro helped move the T480 truck through that sand, over a rutted and rocky trail, and up and down steep hills.

The third vehicle in my sampling was a sleeper-cab T680 pulling a 53-foot van. Its MX-13 diesel/TX-12 transmission powertrain ran like a champ on the grounds’ 1.6-mile paved oval track. I stopped several times to observe it start out smoothly in third and skip-shift through fifth and seventh and then eighth, ninth, 10th and into 11th before I had to slow down from a little over 50 mph for a banked curve. I once forced an upshift to 12th, but this lowered the engine’s revs to an unworkable 800 RPM or so. In these conditions, the electronic controls in all three transmissions knew just what they were doing.

Most impressive was smooth clutch engagement. On level ground, I got to taking it for granted. But on a 15-percent upgrade with the T480 truck, I was astonished: I released the brakes and the clutch took hold with absolutely no shuddering or vibration, and the Cummins-Paccar MX-9 diesel, operating at idle, simply walked us up the hill in first gear. I did this several times to be sure this was normal behavior. Few drivers operating a manual transmission could start out on a hill like this without punishing the clutch. Going down the other side, the TX stayed in a lower gear and worked with the engine brake to keep our speed in check.

Now, an automatic transmission with a torque converter – namely, an Allison – can do this over and over, and it’s not remarkable. Working well in the rough and ease of driving in stop-and-go traffic comprise one reason Allisons have become popular in vocational trucks. Another is that a driver doesn’t have to do much thinking with an Allison; just get on the gas and go. An automated gearbox, however, requires some knowledge for a driver to get the most out of it.

An example was climbing a hill topped by a gravelly dirt trail that ascended at 20 percent. That T880 semi-stormed upgrade with the transmission set in drive, from which it started in second gear and, as gravity’s grip strengthened, paused briefly to try for first but failed. We abruptly stopped. The engine died amid warning buzzers, red lights and some shaking. I hit the brakes, switched to neutral and cranked the engine. Back in drive, I tried to restart on the hill, but traction was lacking and the drivers burrowed into the gravel. “Nope,” Reid and I declared simultaneously, and I meekly backed down the hill.

The powertrain stalled on the hill because the TX can sense what the chassis is doing but cannot see what the truck’s heading into, so the driver must supply the eyes and make an adjustment. Before starting, if I had switched the tranny to manual, then chosen a gear and stayed with it, we’d likely have made it up and over the top. That gear could’ve been second or first – an experienced driver would know – but the key is to keep the controls from attempting an unwise downshift on such a steep hill, just like a good driver would do with a manual gearbox.

Here, too, an Allison wouldn’t have had much problem, except maybe for the loose gravel beneath the drive wheels. It could’ve downshifted under continuous power, maintaining momentum, and its torque converter would’ve cushioned the driveline. But an Allison costs thousands more dollars than an automated transmission and makes it a viable alternative for vocational use. And the feel of power and torque coursing through solid gears makes it more pleasing to drive, at least for me. Just watch what you’re getting into so you don’t get stuck. LL

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